The interaction of Insulin-like Growth Factor -1 (IGF-1) with its own receptor (IGF-1 R) seems to play a major role in normal development and in the control of both normal and abnormal growth. In growth hormone disturbances of growth as, for instance, in acromegalics and in patients with growth hormone deficiency, clinical assessments of disease activity correlate far better with blood levels of IGF-1 than they do with growth hormone concentrations, Van Wyk et al., The Biology of Normal Human Growth, pp. 223-239, Raven Press, N.Y. (1981). Werner et al., Proc. Nat. Acad. Sci. USA, 86:7451-5 (1989) have shown that the mRNA levels for the IGF-1 R decrease steadily in all tissues during post-natal development, reaching a maximum during the perinatal stages. IGF-1 mRNA, instead, is not so tightly regulated during development as the mRNA for the IGF-1 R, and actually reaches maximum expression in the adult liver, which is the main site of production of IGF-1. Apart from these general considerations, a number of reports have appeared indicating that the interaction of IGF-1 with its own receptor may play a major role in cell growth. For instance, IGF-1 receptors are present in phytohemagglutinin activated T lymphocytes, Kozak et al., Cell Immunol., 1009:318-331 (1987) and in K562 cells that are a human erythroleukemia cell line, Hizuka et al., Endocrinol. Japon, 34:81-88 (1987). In fact, K562 cells grow vigorously in serum free media (SFM) containing only IGF-1 or supraphysiological concentrations of insulin. An abundance of IGF-1 receptors has also been reported in lymphoblasts of human T cell leukemias, Lee et al., J. Clin. Endocrinol. & Metabol., 62:28-35 (1986), and in HL60 cells, Pepe et al., J. Cell Physiol., 133:219-227 (1987). In our own laboratory, we have been able to show that the mRNA for the IGF-1 receptor is over-expressed in HL60 cells. Again, HL60 cells, as well as other cell lines, grow well in serum-free medium containing only insulin in supraphysiological concentrations. In Burkitt cells, the number of IGF-1 receptors increase between G.sub.1 and S-3 phase, Hartman et al., Leukemia, 2:241-4 (1988). Stem cells and progenitor cells also seem to require IGF-1 for growth. Goldring and Goldring, Eucar. Gene Express, 1:-301-326 (1991), list several references indicating that IGF-1 increases the proliferation of keratinocytes, smooth muscle cells, osteoblasts, chrondrocyts and neuronal cells (see their Table 4). The IGF- 1 R is induced by estrogens in breast cancer cell lines, Stewart et al., J. Biol. Chem., 265:21172-8 (1990), Pekonen et al., Cancer Res., 48:1343-7 (1988), Peyrat et al., Cancer Res., 48:6429-33 (1988), Foekens et al., Cancer Res., 49:5823-8 (1989), and the expression of IGF-1 receptors seems to correlate with the growth of breast cancer, at least just as well as the estrogen receptors or the EGF receptor. Other tumors in which an increased expression of IGF-1 R or, at least, IGF-1 binding sites, have been reported include small cell lung cancer, Kiefer et al., Exp. Cell Res., 184:396-406 (1989), Minuto et al., Cancer Res., 48:3716-9 (1988), Nakanishi et al., J. Clin. Invest., 82:354-9 (1988), choriocarcinoma cells, Ritvos et al., Endocrinology, 122:395-401 (1988), malignant glioma, Gammeltoft et al., Cancer Res., 48:1233-7 (1988), renal carcinoma, Pekonen et al., Int. J. Cancer, 43:1029-33 (1989), and neoplastic human endometrium, Talavera et al., J. Cancer Res., 50:3019-24 (1990). A role of the IGF-1 R in growth has also been reported in human melanoma cells, Stracke et al., J. Biol. Chem., 264:21544-9 (1989), and in tumors of neural origins like neuroblastomas or pheochromocytomas, Ota et al., Molec. Brain Res., 6:69-76 (1989) and Ota et al., Cur. J. Biochem., 174:521-30 (1988). However, the best evidence that the IGF-1 R plays a major role in the control of cellular proliferation comes from studies with fibroblasts in cell cultures.
The 70 amino acids that comprise the human IGF-1 have been divided into 4 principle domains. The first 29 residues of IGF-1 bear a strong resemblance to the B chain of insulin and, consequently, are known as the B domain. IGF-1 residues 42-62 are homologous to the insulin A chain and hence, known as the A domain. Intervening between the B and A domains (residues 30-41) is the C domain. Finally, the last 7 amino acids (residues 63-70) have been referred to as the D domain. The sequence of IGF-1 is known (SEQ ID NO: 1). Rotwein, P., Pollock, K. M., Didier, D. K., and Krivi, C. C., J. Biol. Chem. 261:4828-4832 (1986) (Sequence translated from the DNA sequence); Jansen, M., van Schaik, F. M. A., Ricker, A. T., Bullock, B., Woods, D. E., Gabbay, K. H., Nussbaum, A. L., Sussenbach, J. S., and Van den Brande, J. L., Nature 306:609-611 (1983) (Sequence translated from the mRNA sequence); Met-24 is proposed as a likely initiator. Rinderknecht, E., and Humbel, R. E., J. Biol. Chem. 253:2769-2776 (1978) (Sequence of residues 49-118).
A detailed solution NMR structure of the core of human IGF-1 was recently reported by Cooke, R. M., Harvey, T. S., Campbell, I. D., Biochem., 30:5484-5491 (1991). The hydrophobic core of IGF-1 is strikingly similar to insulin. In this light, it is interesting to note that, in addition to binding its own type 1 receptor, IGF-1 also binds the insulin receptor, albeit with lower affinity (Massague, J. and Czech, M. P., J. Biol. Chem., 257:5038-5045 (1982)). The most striking structural differences occur between IGF-1 and an insulin dimer because of the inclusion of the C and D domains in the IGF-1 structure. Both the C and D domains were poorly resolved in the structures due to their intrinsic mobility. A molecular model of the human IGF-1 (for general details regarding the building of this molecular model see, Jameson, B. A., Nature, 341:465-466 (1989) that is consistent with the NMR data obtained by Cooke et al. (1991) (supra) has been developed. In this model, the C and D domains appear as "flaps" which flank the insulin-conserved receptor binding cleft (residues 21-24, Cascieri, M. A., Chicchi, G. G., Applebaum, J., Hayes, N. Green, B. C., Bayne, M. L., Biochem., 27:3229-3233 (1988); Bayne, M. L., Applebaum, J., Underwood, D., Chicchi, G. G., Green, B. C., Hayes, N., Cascieri, M. A., J. Biol. Chem., 264:11004-11008 (1989). It is believed that these flaps are directly involved in the specific binding to the type 1 receptor. Consistent with this notion, it has been observed that deletion of the D domain of IGF-1 increased the affinity of the mutant IGF-1 for binding to the insulin receptor, while decreasing its affinity for the type 1 receptor (Cascieri et al., 1988) (supra). Furthermore, some or all of the residues within the C domain, which flank the conserved binding cleft in IGF- 1 but not in insulin, appear to be required for distinguishing between the type 1 and insulin receptors (Bayne et al., (1989) (supra); Cascieri, M. A. and Bayne, M. L., Molecular and Cellular Biology of IGFs and Their Receptors, LeRoth, D. and Raizada, M. K., Eds., Plenum Press (London 1990).